Method for detecting source of error in an amperometric measuring cell

ABSTRACT

The invention is directed to a method for detecting error sources in an amperometric measuring cell 1 which includes at least a measuring electrode 2 and a counter electrode 3 within an electrolyte chamber 4 filled with an electrolyte solution 6. A permeable membrane 7 closes off the electrolyte chamber 4 with respect to the measuring sample. The method includes the steps of: providing a voltage source 10 outputting a voltage U to apply across the electrodes to generate a sensor current i(t) between the electrodes; starting with the voltage U across the electrodes at a reference voltage U 0  with a reference current i 0 , increasing or decreasing the voltage U to a first voltage U 1  during a first time span T 1  ; shortly after the voltage U assumes the first voltage U 1 , measuring a first sensor current i 1  and/or, toward the end of the first time span T 1 , measuring a second sensor current i 2  ; and, comparing the sensor currents i 1  and/or i 2  to the reference current i 0 .

FIELD OF THE INVENTION

The invention relates to a method for detecting error sources in an amperometric measuring cell which includes at least a measuring electrode and a counter electrode in an electrolyte chamber which is closed by a permeable membrane with respect to the measurement sample to be detected. The measuring cell is connected to a voltage source supplying a voltage and generating a sensor current between the electrodes.

BACKGROUND OF THE INVENTION

An electrochemical measuring cell of the above kind is disclosed in U.S. Pat. No. 4,961,834 incorporated herein by reference. In this measuring cell, a measuring electrode, a reference electrode and a counter electrode are arranged in an electrolyte chamber of the measuring cell housing. The electrolyte chamber is filled with an electrolyte and the housing is closed off by a permeable membrane with respect to the measurement sample to be detected. The measuring electrode, the reference electrode and the counter electrode have respective connecting leads which pass through the measuring cell housing and are connected to an evaluation unit having a voltage source. A sensor current i(t) flows after the electrodes are connected to the voltage source.

It is a disadvantage of the known measuring cell that no information can be obtained from the sensor current i(t) as to the state of use of the measuring cell. Thus, the sensor current i(t) can lie within the predetermined limits but precise concentration measurements are no longer possible.

European patent publication 0,419,769 discloses a method for continuously monitoring an electrode system of potentiometric measurement cells wherein symmetrical bipolar current pulses having different period durations are applied repeatedly to the measuring cell. The voltage change caused thereby, referred to the electrode voltage without current pulse, is compared to a desired value determined experimentally or by computer.

It is a disadvantage of this known method that an additional voltage source is necessary with which the check is carried out. It is also disadvantageous that the check must be carried out at different times and with different period durations in order to detect the individual faults.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of detecting error sources for amperometric measuring cells wherein different errors can be detected within a measuring cycle.

The method of the invention is for detecting error sources in an amperometric measuring cell for measuring a sample. The measuring cell includes: an electrolyte chamber having an opening directed toward the sample to be measured and holding an electrolyte; a permeable membrane mounted on the chamber for closing off the chamber; and, a measuring electrode and a counter electrode disposed in the chamber so as to be in spaced relationship to each other. The method includes the steps of: providing a voltage source outputting a voltage U to apply across the electrodes to generate a sensor current i(t) between the electrodes; starting with the voltage U across the electrodes at a reference voltage U₀ with a reference current i₀, increasing or decreasing the voltage U to a first voltage U₁ during a first time span T₁ ; shortly after the voltage U assumes the first voltage U₁, measuring a first sensor current i₁ and/or, toward the end of the first time span T₁, measuring a second sensor current i₂ ; and, comparing the sensor currents i₁ and/or i₂ to the reference current i₀.

The advantage of the invention is seen essentially in that, because of a slight change of the voltage (that is, an increase or a decrease of the voltage to a first voltage U₁ during a first time span T₁), a change of the sensor current i(t) from i₁ to i₂ is generated and that the comparison of the first sensor current i₁ and/or of the second sensor current i₂ to the reference current i₀ is utilized to detect a fault in the measuring cell. For carrying out this measurement, a slight alteration of the voltage is sufficient which lies in a range of approximately 0.02 to 1 millivolt. The first time span T₁ amounts approximately to 100 milliseconds. If the method of the invention is carried out during gassing of the measuring cell with the measurement sample to be detected, then the reference current i₀ is the measurement current and, in a neutral gassing atmosphere, the steady-state sensor base current becomes the reference current.

In an advantageous manner, the voltage is adjusted to a second voltage U₂ during a second time span T₂ directly after the first time span T₁. The second voltage U₂ is directed opposite to the first voltage U₁ compared to the reference voltage U₀. In this way, a polarity reversal is obtained within the measuring cell and the reference current i₀ adjusts immediately at the end of the second time span T₂ at the measuring cell.

The second time span T₂ is so selected that it is equal to or less than 1.5 times the first time span T₁.

The second time span T₂ is defined by the equation:

    T.sub.2 =T.sub.1 ×ln(1-Y×(1-1/X))/ln(X)

wherein:

    X=(i.sub.1 -i.sub.0)/(i.sub.2 -i.sub.0) and

    Y=(U.sub.1 -U.sub.0)/(U.sub.2 -U.sub.0).

The parameters C_(m) and G_(m), which characterize the measuring cell, are computed in accordance with the following equations:

    G.sub.m =(i.sub.1 -i.sub.0)/(U.sub.1 -U.sub.0)

    C.sub.m =T.sub.1 ×G.sub.m /ln((i.sub.1 -i.sub.0)/(i.sub.2 -i.sub.0)).

The parameters C_(m) and G_(m) can, for example, be compared to input values C_(m0) and G_(m0) and, when a previously determined limit value is exceeded, an indication is provided that the measuring cell is consumed or damaged and must be exchanged for a new one.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings wherein:

FIG. 1 is a schematic of an amperometric measuring cell having two electrodes;

FIG. 2 shows the sensor current i(t) plotted as a function of time when voltages U₁ and U₂ are sequentially applied;

FIG. 3 is an equivalent circuit of the measuring cell of FIG. 1; and,

FIG. 4 is an equivalent circuit of a measuring cell incorporating a reference electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a schematic configuration of an electrochemical measuring cell 1 having a measuring electrode 2 and a counter electrode 3. The electrodes (2, 3) are arranged in an electrolyte chamber 4 of a housing 5 of the measuring cell 1. The measuring cell housing 5 is filled with an electrolyte 6 in the form of an aqueous solution and is closed off with respect to the gas sample to be detected by a permeable membrane 7. The electrodes (2, 3) are connected via lines (8, 9) to a voltage source 10. A voltage U is applied across the electrodes (2, 3) by means of the voltage source 10. The sensor current i(t) is tapped off as a voltage drop across a measurement resistor 11 in the line 9.

FIG. 2 shows the sensor current i(t) as a function of time (t) and dependent on the voltage U(t). The voltage U is increased to the first voltage U₁ during a first time span T₁ causing the sensor current i(t) to increase from the reference current i₀ to the first sensor current i₁ and then drop within the first time span T₁ to the second sensor current i₂.

A second time span T₂ follows directly after the first time span T₁. During the second time span T₂, the voltage is reduced to the second voltage U₂ and the sensor current i(t) drops relative to the reference current i₀ and becomes the reference current i₀ after the second time span T₂. The sensor currents i₀, i₁ and i₂ are read into an evaluation unit (not shown) which contains a microprocessor which compares the sensor currents and carries out the computation operations.

The evaluation unit furthermore controls the changes of the voltage from U₀ to U₁ and from U₁ to U₂ and from U₂ to U₀. The first voltage U₁ is adjusted in such a manner that it lies approximately 0.02 to 1 mV above the reference voltage U₀ and the duration of the first time span T₁ is approximately 100 milliseconds. The duration of the second time span T₂ is adjusted in such a manner that it amounts to approximately 0.2 to 1.5 times the first time span T₁.

The second time span T₂ can also be computed from the measured sensor currents i₀, i₁ and i₂ based on a simplified equivalent circuit diagram shown in FIG. 3.

The measuring cell 1 of FIG. 1 can be defined electrically by a measuring electrode capacitor C_(m) and a measuring electrode conductance value G_(m). The measuring electrode capacitor C_(m) is conjointly defined by the measuring electrode 2 and the counter electrode 3 together with the electrolyte 6 disposed therebetween. The measuring electrode conductance value G_(m) indicates the ohmic resistance between the electrodes (2, 3) and the contact resistances between the electrodes (2, 3) and the leads (8, 9).

The second time span T₂ can be computed from the following:

    T.sub.2 =T.sub.1 ×ln(1-Y×(1-1/X))/ln(X)

wherein:

    X=(i.sub.1 -i.sub.0)/(i.sub.2 -i.sub.0) and

    Y=(U.sub.1 -U.sub.0)/(U.sub.2 -U.sub.0).

The measuring electrode capacitance C_(m) and the measuring electrode conductive value G_(m) are computed from the following formulas:

    G.sub.m =(i.sub.1 -i.sub.0)/(U.sub.1 -U.sub.0)

    C.sub.m =T.sub.1 ×G.sub.m /ln((i.sub.1 -i.sub.0)/(i.sub.2 -i.sub.0)).

Desired or set values for the measuring electrode capacitance and the measuring electrode conductance are stored in the evaluation unit as reference measuring electrode capacitance C_(m0) and as reference measuring electrode conductance value G_(m0). Within the evaluation unit, a comparison is carried out between the computed parameters C_(m) and G_(m) with the desired values C_(m0) and G_(m0).

Deviations of the parameters C_(m), G_(m) from the desired values C_(m0) and G_(m0) can have the causes delineated below. Thus, a defective contact of the measuring electrode, for example, affects only the measuring electrode conductance value G_(m) ; whereas, a decreasing wetting of the measuring electrode (for example, because of drying out) becomes manifest primarily in the measuring electrode capacitance C_(m). The tolerance limits for C_(m) and G_(m) can be selected to be relatively narrow because the temperature dependency of G_(m) and C_(m) is easily determined and can be linearly approximated over a wide range. In this way, not only can a complete failure of the sensor be detected but also changes can be detected which would lead to a failure only later or which would impermissibly affect the measuring characteristics of the sensor.

The method of the invention for detecting faults is also applicable to a three-electrode measuring cell 12 having a reference electrode. The equivalent circuit diagram of such a sensor is presented in FIG. 4 wherein the same components are identified by the same reference numerals used in FIGS. 1 and 3.

The reference electrode (not shown in FIG. 4) is connected to a line 13. In the equivalent circuit diagram of FIG. 4, G_(g) identifies the conductance value of the counter electrode, C_(g) the capacitance of the counter electrode, G_(r) the conductance value of the reference electrode and C_(r) the capacitance of the reference electrode. The conductance values can be expressed physically as follows: resistance of the input line to the electrode, transfer resistance of the contact between input line and the electrode and transfer resistance between electrode and electrolyte; and, the capacitors are double layer capacitors between the electrodes.

It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. A method for detecting error sources in an amperometric measuring cell for measuring a sample, the measuring cell including: an electrolyte chamber having an opening directed toward the sample to be measured and holding an electrolyte; a permeable membrane mounted on said chamber for closing off said chamber; and, a measuring electrode and a counter electrode disposed in said chamber so as to be in spaced relationship to each other; and, the method comprising the steps of:providing a voltage source outputting a voltage U to apply across said electrodes to generate a sensor current i(t) between said electrodes; starting with said voltage U across said electrodes at a reference voltage U₀ with a reference current i₀, increasing or decreasing said voltage U to a first voltage U₁ during a first time span T₁ ; shortly after said voltage U assumes said first voltage U₁, measuring a first sensor current i₁ and/or, toward the end of said first time span T₁, measuring a second sensor current i₂ ; and, comparing said sensor currents i₁ and/or i₂ to said reference current i₀, wherein the comparison of the sensor currents i₁ and/or i₂ to the reference current i₀ detects a fault in the measuring cell.
 2. The method of claim 1, further comprising the step of adjusting said voltage U to a second voltage U₂ during a second time span T₂ directly after said first time span T₁ with said second voltage U₂ being directed opposite to said first voltage U₁ relative to said reference voltage U₀.
 3. The method of claim 2, wherein said second time span T₂ is equal to or less than 1.5 times said first time span T₁.
 4. The method of claim 2, said second time span T₂ being defined by the equation:

    T.sub.2 =T.sub.1 ×ln(1-Y×(1-1/X)/ln(X)

wherein:

    X=(i.sub.1 -i.sub.0)/(i.sub.2 -i.sub.0); and,

    Y=(U.sub.1 -U.sub.0)/(U.sub.2 -U.sub.0).


5. The method of claim 1, wherein the measuring cell can be defined by an equivalent circuit including parameters C_(m) and G_(m) wherein:

    G.sub.m =(i.sub.1 -i.sub.0)/(U.sub.1 -U.sub.0); and,

    C.sub.m =T.sub.1 ×G.sub.m /ln((i.sub.1 -i.sub.0)/(i.sub.2 -i.sub.0)).


6. The method of claim 5, further comprising the step of comparing said parameters C_(m) and G_(m) to desired or set values C_(m0) and G_(m0), respectively. 